Crop growth and nitrogen turnover under increased temperatures and low autumn and winter light intensity

The rise in mean annual temperatures under the projected climate change will affect both soil organic matter turnover and cropping patterns in agriculture. Nitrogen (N) mineralization may be higher during autumn and winter and may increase the risk of nitrate leaching. Our study tested whether a soil cover of winter wheat or a ryegrass catch crop would be able to take up the extra N mineralized during autumn and winter under the low light conditions in Northern Europe, both at current average temperatures (T0) and at 4 °C (T+4) and 8 °C (T+8) above average. The crops were grown in pots in growth chambers from mid-September to February. Two sowing times were included for winter wheat, with an early sowing in September representing current practice and a late sowing in October to reflect possible future cropping conditions. Biomass yield and soil inorganic N were determined from replicate pots in November, December and February. Reference pots with bare soil were included. N mineralization clearly increased with higher temperatures with, respectively, 22% and 80% more N mineralized in bare soil at T+4 and T+8 than at T0 after 136 days. The ryegrass catch crop emptied the soil of inorganic N very efficiently during both autumn and winter months at all temperatures. The early-sown wheat crop left negligible amounts of inorganic N in the soil at the last harvest in February at T+4 and T+8, whereas it was unable to take up all mineralized N at T0. The prolonged period without a crop before the late sowing of wheat caused generally higher levels of inorganic N to accumulate in soil. Despite the higher mineralization under the raised temperatures, at T+8 the late-sown winter wheat was able to reduce soil inorganic N to a lower level than late-sown wheat at the two lower temperatures. The results indicate that even at the low light intensity during autumn and winter, accelerated N mineralization caused by raised temperatures can potentially be offset by crop N uptake. However, sowing should take place sufficiently early in the autumn to give the crop time to capture the additional N mineralized during autumn, and this may collide with the projected need for later sowing of winter cereals under a warmer climate.